Vacuum tubes were developed at or around the turn of the century and immediately became widely used for electrical amplification, rectification, oscillation, modulation, and wave shaping in radio, television, radar, and in all types of electrical circuits. With the advent of the transistor in the 1940s and 1950s and integrated circuit technology in the 1960s, the use of the vacuum tube began to decline, as circuits previously employing vacuum tubes were adapted to utilize solid state transistors. The result is that today more circuits are utilizing solid state semiconductor devices, with vacuum tubes remaining in use only in limited circumstances such as those involving high power, high frequency, or severe environmental applications. In these limited circumstances, solid state semiconductor devices generally cannot accommodate the high power, high frequency or severe environmental conditions.
There have been a number of attempts at fabricating vacuum tube devices using solid state semiconductor device fabrication techniques. One such attempt resulted in a thermionic integrated circuit formed on the top side of a substrate, with cathode elements and corresponding grid elements being formed co-planarly on the substrate. The anodes for the respective cathode/grid pairs were fabricated on a separate substrate, which was aligned with the first-mentioned substrate such that the cathode to anode spacing was on the order of one millimeter. With this structure, all the cathode elements were collectively heated via a filament heater deposited on the backside of the substrate. Accordingly, this structure required a relatively high temperature to operate and required substrate materials with a high electrical resistivity at elevated temperatures. In addition, the structure described above presented other problems, including: inter-electrode electron leakage, electron leakage between adjacent devices, and a limited cathode life.